The field of application of rare earth permanent magnetic materials is extremely extensive. They are already being widely used in electronic instruments, mechanical and electrical machines, metallurgy, petroleum, telecommunication, aviation and astronavigation, military and medical industries.
Since Strnat and Hoffer discovered the magnetic anisotropy of yttrium cobalt compound YCo.sub.5 in 1966, worldwide continuing efforts have been made on the exploration of better rare earth permanent magnetic materials with better magnetic properties. Tremendous progress was then made in the last few decades and was marked by the three milestones. RCo.sub.5 based magnets with its maximum energy product of around 20 MGOe were referred to as the first generation magnets, R.sub.2 Co.sub.17 type magnets with their maximum energy product of around 30 MGOe were referred to as the second generation magnets and the NdFeB type magnets with their maximum energy product of about 50 MGOe or over as announced in 1983 were referred to as the third generation magnets.
The emergence of NdFeB magnets with their amazing magnetic properties and very low cost of production has made feasible many applicaitions which would otherwise be impossible and has substantially promoted the miniaturization of many devices or instruments. However, NdFeB magnets are not without shortcomings. Low curie point, low thermal stability and rusting often place limits to their use. SmCo magnets are superior to NdFeB in these aspects but not costwise. Therefore to develop low cost SmCo magnets enabling it to survive the competition with NdFeB has been one of the current trends of research and development work.
The object of this invention is therefore to provide a modified reduction and diffusion process to reduce the production cost of the alloy of the R.sub.2 Co.sub.17 type.
Reduction and diffusion method has long been reported for the preparation of SmCo5 alloy powder (U.S. Pat. No. 3625779, JP79-102271, JP79-87630). Since the very expensive samarium metal is no longer required and the alloy powder may be obtained without melting and crushing operations, the cost of processing is substantially lower. However, such a method is workable only for the simple binary alloy, SmCo.sub.5, and the composition control would be very difficult for multicomponent systems of the R.sub.2 Co.sub.17 type.
In order to obtain better coercivity for 2:17 alloys, copper must be added and calcium is liable to form alloy with copper thereby makig less copper available for the enhancement of coercivity.
Thus the difficulties associated with the preparation of SmCo 2:17 alloy powder are as listed below.
1. In order to compensate for the amount of copper alloyed with calcium, an excess of copper must be added. But the more copper is added, the more calcium would be consumed, thereby making the production cost higher.
2. Due to the formation of Ca-Cu alloy, the consumption of Ca and Cu would be increased. Therefore the difficulties associated with the removal of Ca and Cu in the finishing processes would accordingly be increased.
3. For reasons as stated above, the amount of metallic calcium available in the reduction process cannot be precisely controlled leading to incomplete reduction of samarium oxide. Therefore the composition of the final product often deviates from the target composition making it difficult to attain target magnetic properties.
4. In order to ensure complete reduction and diffusion reactions, the diffusion temperature is often required to go beyond 1200.degree. C. Excessively high temperature not only shortens the service life of equipment but also tends to cause excessive loss of metal samarium in the molten state (m.p. 1072.degree. C.) during its diffusion process.
5. Not infrequently, hard cakes are formed in the reaction mixture that require crushing prior to leaching.
Entrainment of extraneous matters cannot be fully released due to unsatisfactory particle size distribution. Oxygen content of the final product may exceed 0.2% or sometimes even 0.5%. As is known by all, excessive oxygen content in the magnetic alloy powder is detrimental.